1. Field of the Invention
The present invention relates to a filter to be employed for filtration of fine substances such as filtration for sterilization, purification of crystals and the like (such filtration as the above will be referred to as "Semi-ultrafiltration" hereinafter), and to a process for manufacturing the same.
2. Related Art Statement
As conventional filters, there are known ceramic filters formed of sintered fine ceramic particles or a porous sintered ceramic material having its surface covered with fine ceramic particles, as shown in Japanese Patent Application Publication No. 8,643/81, or membrane filters comprising a filter membrane of a resinous material.
While ceramic filters are excellent in corrosion-resistibility and durability, such filters put to use have generally a limited pore size of about 0.8.mu. at the smallest from restrictions of the starting particle size and sintering conditions. Accordingly, ceramic filters are not employable for semi-ultrafiltration such as filtration for sterilization, purification of crystals and the like, so that their uses are extremely limited.
In contrast with the above, membrane filters, as represented by those of polytetrafluoroethylene, can be provided with micropores of a pore size as small as about 0.1.mu.. These membrane filters exhibit an improved durability and therefore are suitable for the semi-ultrafiltration. However, membrane filters possess a relatively small tensile strength per unit area as compared with ceramics or metals and soften as the ambient temperature is raised, thus resulting in the loss of tensile strength. Further, if being placed under a tensile stress even at a temperature lower than the melting point temperature of the membrane for a long period of time, the membrane will undergo a large thermal transformation The thermal transformation causes the membrane to undergo creep which results in a permanent deformation, thus causing its pore size to change, and therefore, will be inconvenient to utilize by regeneration.